Geophysicists have captured, for the first time, the ocean floor in the act of separating at one of its seams. Using a set of more than 20 measuring stations placed in a 100-kilometer-long region of the Indian Ocean, they witnessed an event that released around 160 million cubic meters of lava to the sea floor and displaced two sections of oceanic crust by at least 2 meters in a matter of days.

The magnitude of the event, which was described today in Nature¹It was “a big surprise,” according to study co-author Jean-Yves Royer, a marine geophysicist at the French National Center for Scientific Research (CNRS) in Brest.

The oceanic crust covers almost two-thirds of the planet and the underwater ridges in the middle of the ocean are responsible for its creation. As the movement of tectonic plates separates the existing crust from the ridge, magma erupting from the Earth’s core produces new crust and solidifies. The process has been widely understood since the mid-20th century, but has never been observed in real time.

Despite the critical role of mid-ocean ridges in shaping the Earth’s surface, “we still know very little about the frequency, magnitude and dynamics of eruptions and the tectonic processes that generate them,” says Isobel Yeo, a geoscientist at the National Oceanography Center in Southampton, UK.

Stress release

Royer and his colleagues focused on India’s Southeastern Ridge, which runs across the floor of the Indian Ocean in a roughly east-west direction. The ridge separates the Antarctic plate from the Australian plate (which contains Antarctica and Oceania as well as the oceanic crust surrounding them).

In total, the two plates move away from each other about six centimeters a year, mainly because the Australian plate is dragged northwards. But some sections of the Australian plate could remain stationary for a while and then undergo a huge burst of motion, accompanied by earthquakes. Hoping to capture one such event, in February 2024, the team placed three types of instruments in various locations around a 100-kilometer-long segment of the ridge.

In particular, they deployed five hydrophones, which are underwater microphones that can detect sound waves, including those produced by earthquakes. The team also installed 15 acoustic beacons, which are battery-powered stations on stilts that can emit and detect underwater sounds. Every four hours, the beacons exchange sound signals and measure the time it takes for the response to arrive.